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Original question:

LMP7721: Transimpedance amplifier - Question about layout, guard and copper pours

LMP7721: OPA928 and LMP7721: feedback cap

Richard Payne
Prodigy 30 points
Part Number: LMP7721


Tool/software:

I have a trans-impedance circuit with 100M and 2G feedback. I was required to add a capacitor in a limited space. the value would be 1300pF (or 2700pf) and 560pF. 

I usually select polystyrene, which has been popular in the past but is no longer available due to low-temperature soldering requirements. It must meet ROHS, and 96 solder must be used. 

I could use polypropylene, but it seems bulky.

I'm open to suggestions on the best feedback capacitor that is compact, has good availability and has low leakage. Is MICA any good?

R. 

 

  • 0
    TI__Guru 62386 points

    Hello Richard,

    Mica actually is not too bad leakage-wise (being a high-voltage device), but being crystalline based, is very microphonic - so any stresses or vibrations will create a charge (voltage). We always used the Sprague/CDE 600V red molded leaded micas. Tek and HP also used these, too. If you leave a little lead length - a few mm - that helps to releave the stress on the package.

    Polystyrene tends to have some triboelectric effects - any stresses will create a voltage (think soda bottle).

    Polypropylene is the next best, but are larger due to the lower volume coefficient.

    Teflon/PTFE is the best, but tend to be in smaller values (<100pF) and larger sizes as they tend to be used in microwave power amplifiers.

    My findings were that the lowest leakages were in the higher voltage (>200V) caps. Most of the tantalum and ceramic based caps are very leaky, particularly as you get to smaller sizes with lower voltages (with exotic dielectric formulations). Simple is better...and that's why the best caps are larger.

    It's been years since I did the low leakage stuff...so I am not up on the "latest' capacitor technology. I would reach out to the capacitor manufacturers.